When laying a pipeline at sea it is customary to weld, on a lay-barge, individual pipe sections into a pipe string (with the pipe string leading towards the seabed). The pipe-string is under great tension while being laid and weld joints must be sufficiently strong to withstand the high forces imposed on the weld joints. Each time a pipe is welded to another pipe, extensive tests are made to ensure that the quality of the weld joint formed is sufficient. The strength of a weld joint depends upon various factors, one being the geometry of the path traced by the point of contact of the arc in relation to the surfaces of the pipes to be joined. If the point of contact of the arc is not closely controlled, the quality of the weld may be insufficient.
One conventional method of welding two pipes together involves beveling the ends of the pipes such that when the pipes are arranged coaxially with respect to each other immediately before the welding process commences, an exterior circumferential groove is defined between the two pipes. A carriage is mounted on one of the pipes for movement around the circumference of the pipes to be joined, e.g., a conventional “bug & band” system. A welding torch is mounted on the carriage and the apparatus is so arranged that the end of the metal electrode of the torch is opposite and relatively close to the circumferential groove. The carriage is moved around the circumference of the pipe and the torch is operated so that an arc is directed into the groove. The arc is guided manually and/or by various mechanical sensors to guide the arc as accurately as possible along the length of the groove. The welding process generally takes several passes and often the passes are performed by a series of sequentially positioned welding stations on a lay-barge.
In pipe laying, the welding process is highly repetitive requiring the welding of hundreds of pipe joints per day with multiple passes on each pipe joint. Therefore, even small increases in the speed of the individual welding cycle for each pipe joint can result in significant operational savings.